This invention relates to shaft revolution counters in general, and in particular to electronic shaft revolution, and other event, counters that do not need an external power source, and do not employ moving mechanical parts.
Revolution counters, in general, can be divided into two categories:
1. Bi-directional e.g. for multi-turn shaft angle encoders.
2. Uni-directional e.g. for recording the consumed quantity in a water meter.
Multi-turn shaft angle encoders are used where events (shaft revolutions) must be registered even when no power is available. For example, when a lead screw in an idle CNC machine is manually rotated, but the position of the driven load must be known when power is restored.
Prior art multi turn encoders usually employ a step-down gear mechanism followed by a single-turn encoder together making a non-volatile mechanical memory. It has been a long-standing desire to provide electronic non-volatile shaft revolution counters without a mechanical gear or a backup battery powering an electronic counter. Consequently, attempts have been made to tap on the motion of the rotating shaft in order to generate the electrical energy needed for the write process in an EEPROM.
German patents: DE 43 42 069 A1, DE 43 42 069 C2 (U.S. Pat. No. 5,565,769) and DE 44 13 281 C2 (U.S. Pat. No. 5,714,882) describe a bi-directional shaft revolution counter mechanism for a multi turn encoder that records shaft revolutions with no external power source and which responds even to very slow shaft rotations. It is based on a first permanent magnet pivoted between a stationary iron cored coil and the monitored shaft, and a second magnet that is attached to the monitored shaft. When the monitored shaft is rotating the first magnet is attracted to the second magnet and then snaps back. The abrupt motion induces a voltage pulse in the stationary coil, which is sufficient to operate a non-volatile counter comprising an EEPROM and a micro controller. This Electro-mechanical approach has several drawbacks:
Mechanical complexity combined with sensitivity to vibrations.
Appreciable torque disturbances to the monitored shaft in torque sensitive applications, such as water meters, due to interaction forces between the two magnets.
An alternative approach for counting shaft revolutions is based on tapping energy from the rotating shaft by means of a Wiegand sensor in combination with a magnet attached to the rotating shaft. However the energy content of such spikes is orders of magnitude lower than needed to energize EEPROMS and is marginally sufficient for energizing the newer Ferro electric memories, which are known to consume very low powerxe2x80x94see U.S. Pat. No. 6,084,400. However, the energy content in the generated pulses is unstable which may lead to miscounts. The present invention employs a conventional reed relay in a novel manner to serve as a reed magnetic field sensor.
It is an object of the present invention to provide uni-directional and bi-directional electronic non-volatile shaft revolution, and other event, counters energized by reed magnetic sensors.
It is another object of the present invention to provide unidirectional and bi-directional shaft revolution counters with minimal torque disturbance to the rotating shaft. Further objects and advantages of the invention will become apparent as the description proceeds.
Reed relays comprise a coil and reed contact elements that close and open in response to magnetic field generated by current passage in the coil. These relays are known for many years and are conventionally used for making and breaking electrical contacts. The present invention is based on a novel use of the reed relay, referred to as a reed magnetic sensorxe2x80x94wherein it serves for detecting magnetic fields with no regard to its electrical contacts. The novel application is based on an interaction between a reed magnetic sensor and ambient magnetic field, typically due to rotating magnet. This induces voltage spikes in the coil that both energize a non-volatile electronic counter and are counted by it.
According to a preferred embodiment of the invention a bidirectional shaft revolution counter includes an assembly comprising a magnet mounted on the rotating shaft, and three stationary reed magnetic sensors facing the magnet and generating voltage spikes in response to said shaft rotation. The voltage spikes are applied to an electronic state machine and counter that typically includes Ferro-electric non-volatile logic elements. A further preferred embodiment of the invention suitable for water meters is a uni-directional revolution counter with a single stationary reed magnetic sensor facing said magnet.
A still further object of the invention is to provide a rotation sensing based on detecting the teeth of a ferromagnetic tooth gear.